A ball made of ceramic (ceramic sphere) used as the rolling element or the like in a rolling bearing is fabricated by molding raw material powder to form an unsintered ball of ceramic, i.e. a green ball, sintering the same, followed by a grinding process to finish a ceramic sphere having a shape close to a true sphere. However, there is a problem that the processing is time consuming due to the extremely high hardness of the ceramic sphere.
A green ball prior to sintering is readily worked by virtue of its low hardness. A ball can be processed with efficiency significantly higher than that of grinding after sintering. In view of the foregoing, various proposals have been made in the working of a green ball (for example, refer to Japanese Patent Laying-Open No. 1-130908 (PTL 1), Japanese Patent Laying-Open No. 2-303767 (PTL 2), and Japanese Patent Laying-Open No. 7-314308 (PTL 3). PTL 1 discloses a method including the steps of sandwiching a green ball having a composition based mainly on thermoplastic organic polymer compound added as a binder between two plates facing each other, dropping abrasives and water down onto the grinding plate to carry out wet type grinding. However, this method is disadvantageous in that the composition of the binder is restricted. This method also has a problem in mass productivity since the degreasing and sintering conditions after processing are complex. PTL 2 discloses a method of grinding by sandwiching a green ball between two vertical grindstones. However, this problem has a problem in mass productivity since a plurality of green balls cannot be worked simultaneously by one apparatus.
PTL 3 discloses a method of grinding, including the steps of sandwiching a plurality of green balls between a pair of working plates that are flat and face each other, and grinding the green balls to a shape close a true sphere while causing each of the green balls to rotate in orbital motion and around its own axis in self-rotation in various directions by the relative displacement of a plurality of lines along the opposite planes of the two working plates. A relative displacement of a plurality of lines implies rotation of the two working plates decentered relative to each other, a combination of rotation and straight forward advancement, and the like. According to the working apparatus, the rough surface directed to grinding at the lower working plate can be formed of an open-meshed rough faced constituent member having a mesh that allows passage of ground particles, and a plurality of holes that open upwards can be provided at the lower working plate region of the rough faced constituent member. This method does not have the component of the binder restricted, and allows processing of a plurality of balls at the same time.